Rotor system vibration absorber

ABSTRACT

A rotor system vibration absorber for use with a helicopter of other rotorcraft is disclosed in which spring forces are provided by a plurality of elongated rods arranged in a selected pattern. The rods are coupled at one end to a fixed base that is coupled to a rotor hub, and at the other end to a tuning weight.

TECHNICAL FIELD

The present invention relates to vibration absorbers. In particular, thepresent invention relates to rotor hub vibration absorbers forhelicopters and other rotorcraft.

DESCRIPTION OF THE PRIOR ART

Rotor induced vibration is a major environmental factor in helicopteroperations. The main source of rotor induced vibration is the inherentexcitation caused by transverse airflow into the rotating wing. Whileevery effort is made during the design stage to overcome this problem bycareful design of the rotor and fuselage, it is sometimes necessary toemploy parasitic devices, such as vibration absorbers, to reduce thisrotor system vibration. One such method is to install vibrationabsorbers at the rotor hub. By installing vibration absorbers at therotor hub, the inherent rotor excitation caused by the transverseairflow into the rotor can be minimized at the source of the problem.

A common form of rotor head vibration absorber is the pendulum, bothstandard and bifilar, which is generally installed above the rotor head.These devices are “planar” devices that can counteract hub shears in thesame plane. Although these devices may be effective overall, a largeportion of their installed weight does not contribute to absorbingvibration, thus making that weight ineffective. Furthermore, pendulumsrequire pivot bearings that require maintenance. Another drawback tobifilar pendulum designs is that their operation relies upon slidingand/or rolling metal surfaces, which is not desirable from reliabilityand maintenance points of view.

Referring to FIG. 1 in the drawings, a prior-art vibration absorber 11is illustrated. Vibration absorber 11 is disposed above a rotor head 13,and is covered by a fairing 14. Vibration absorber 11 includes a tuningweight 15 that pivots about a ball joint 17 coupled to the lower end ofa barrel 19 that is disposed within a rotor mast 21. Ball joint 17 isprotected from debris by a protective boot 27. Tuning weight 15 isbiased in line with a main rotor center axis 23 by three springs 25,each having a spring rate K. Springs 25 allow tuning weight 15 to flapin all directions in a plane perpendicular to axis 23. Vibrationabsorber 11 employs various moving parts. The operation of vibrationabsorber 11 relies upon the proper functioning of all three springs 25and ball joint 17, which provides vertical retention. Because tuningweight 15 only moves in a single plane, vibration absorber 11 onlycounteracts in-plane hub shear forces.

Referring now to FIGS. 1B and 1C in the drawings, another prior-artvibration absorber 31 is illustrated. Vibration absorber 31 is disposedabove a rotor head 33. Vibration absorber 31 includes a plurality ofspirally wrapped fiberglass spring arms 35. The inner ends of springarms 35 are coupled to a mast 37, and the outer ends of spring arms 35are coupled to a moving weight 39. Although vibration absorber 31 hasfewer moving parts than vibration absorber 11, vibration absorber 31 isvery complex. Just as with vibration absorber 11, a significant portionof the weight of vibration absorber 31 is ineffective at absorbing rotorsystem vibration, and vibration absorber 31 is only capable ofcounteracting in-plane hub shear forces.

Although the foregoing designs represent advances in the area of rotorhub vibration absorption, significant shortfalls remain.

SUMMARY OF THE INVENTION

There is a need for a rotor system vibration absorber for use on ahelicopter or other rotorcraft that can be installed above and/or belowthe rotor hub for minimizing vibration due to both in-plane hub shearforces and out-of plane hub bending moments, and that requires little orno maintenance.

Therefore, it is an object of the present invention to provide aweight-efficient rotor system vibration absorber for use on a helicopteror other rotorcraft that can be installed above and/or below the rotorhub for minimizing vibration due to both in-plane hub shear forces andout-of-plane hub bending moments, and that requires little or nomaintenance.

The above object is achieved by providing a rotor system vibrationabsorber having a simple, low-cost design in which a plurality ofelongated rods are arranged in a selected pattern. Each rod is coupledat one end to the rotor hub, and at the opposing end to a tuning weight.

The vibration absorber of the present invention provides the followingsignificant advantages over the prior art. The vibration absorberaccording to the present invention has a simple, low-cost design havingno moving parts. This feature significantly reduces wear andmaintenance. Each rod provides an independent load path, thereby makingthe system fail safe. In the present invention, over 80% of the weightof the vibration absorber is utilized as a tuning weight, therebyeliminating the weight inefficiencies present in prior-art devices. Thevibration absorber of the present invention can be installed aboveand/or below the rotor hub. This allows it to counteract not onlyin-plane hub shear forces, but out-of-plane hub bending moments, i.e.,roll and pitch. The rotor system vibration absorber of the presentinvention is easily maintainable in the field because it is has a highlevel of reliability and failures are easily detectable.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. However, the invention itself, as well asa preferred mode of use, and further objectives and advantages thereof,will best be understood by reference to the following detaileddescription when read in conjunction with the accompanying drawings,wherein:

FIG. 1A is a cross-sectional view of a prior-art vibration absorber;

FIG. 1B is a cross-sectional view of another prior-art vibrationabsorber;

FIG. 1C is a top plan view of the prior-art vibration absorber of FIG.1B;

FIG. 2 is a perspective view of one embodiment of a rotor system with avibration absorber according to the present invention;

FIG. 3 is an enlarged cut-away view of the vibration absorber of FIG. 2;

FIG. 4 is a schematic of one of the rods of the vibration absorber ofFIG. 3 shown in a deflected mode;

FIG. 5 is a free body diagram of the rod of FIG. 4;

FIG. 6 is the derivation of the equation for the spring rate of the rodof FIG. 5;

FIG. 7 is the derivation of the equation for the bending stress of therod of FIG. 5;

FIG. 8 is a schematic of the rod and base plate of FIG. 3;

FIG. 9 is a free body diagram of the rod and base plate FIG. 8;

FIG. 10 is the derivation of the equation for the minimum width of theend plate of FIG. 9;

FIG. 11 is a plot showing the vibratory mode shape of the drive mast ofthe rotor system of FIG. 2;

FIG. 12 is a top plan view schematic showing the preferred arrangementof rods about a rotor hub for the preferred embodiment of the presentinvention;

FIG. 13 is a side elevation view in partial cross section of thepreferred embodiment of a rotor system with a vibration absorberaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 2 in the drawings, one simplified embodiment a rotorsystem 53 with a vibration absorber 51 according to the presentinvention is illustrated. Vibration absorber 51 counteracts rotorinduced vibration from rotor system 53 of a helicopter or otherrotorcraft. In particular, vibration absorber 51 is a spring-mass systemthat is dynamically tuned to reduce the principal blade-passagefrequency vibration in rotor system 53. Vibration absorber 51 is capableof counteracting both in-plane forces and out-of-plane moments withoutthe use of complicated moving parts. The in-plane forces are vibratoryshear forces that generally act in the plane of a rotor hub 55, and theout-of-plane moments are generally vibratory bending moments about axesthat lie in the plane of hub 55, i.e., moments caused by roll and pitch.The plane of hub 55 will be referred to herein as the rotor plane.

The present invention may be utilized in any of the followingimplementations: (1) with a single vibration absorber 51 mounted abovehub 55 such that the tuning weight is disposed above hub 55; (2) with asingle vibration absorber 51 mounted below hub 55 such that the tuningweight is disposed below hub 55; (3) with one vibration absorber 51mounted above hub 55 such that the tuning weight is disposed above hub55, and a second vibration absorber 51 mounted below hub 55 such thatthe tuning weight is disposed below hub 55; (4) with a single vibrationabsorber 51 mounted below hub 55 such that the tuning weight is disposedabove hub 55; and (5) with a single vibration absorber 51 mounted abovehub 55 such that the tuning weight is disposed below hub 55. FIG. 2illustrates implementation (3) in which one vibration absorber 51 ismounted above hub 55 such that the tuning weight is disposed above hub55, and a second vibration absorber 51 is mounted below hub 55 such thatthe tuning weight is disposed below hub 55. Implementation (4) as shownin FIGS. 12 and 13 represents the preferred embodiment of the presentinvention, i.e., a single vibration absorber 51 mounted below hub 55such that the tuning weight is disposed above hub 55.

Rotor system 53 includes hub 55, a plurality of variable pitch rotorblades (not shown) that are hingedly coupled to hub 55 at yoke lugs 57,and pitch control assemblies 59 that are disposed between hub 55 andeach rotor blade to control the pitch of the rotor blades through theuse of pitch links 60. Engine torque from a conventional drive means(not shown) is transferred to rotor system 53 through a drive mast 61,such that hub 55 drives the rotor blades.

Referring now to FIG. 3 in the drawings, vibration absorber 51 isillustrated in a cut away view. Vibration absorber 51 may be formed fromtwo halves that are mirror images of each other. In this view, only onesuch half of vibration absorber 51 is shown for clarity. The assembly ofthe two halves of vibration absorber 51 will be discussed in more detailbelow. Vibration absorber 51 includes a base portion 71, a plurality ofrods 73, a top portion 75, an annular adapter ring 77, an annular disk79, and one or more annular tuning weights 81. It will be understoodthat top portion 75, annular adapter ring 77, annular disk 79, and/ortuning weight 81 may be integrally combined to form a single component.Vibration absorber 51 preferably has a generally cylindrical shapehaving with a central longitudinal axis 52. Tuning weight 81 is locatedat a radius R1 from central axis 52.

Base portion 71 includes a plurality of mounting flanges 83 havingmounting apertures 85 through which pass fasteners (not shown) forcoupling vibration absorber 51 to either the top or bottom of hub 55. Inaddition, each half of base portion 71 includes attachment flanges 87 aand 87 b for coupling the two halves of vibration absorber 51 together.Both attachment flanges 87 a and 87 b have an attachment aperture 89through which passes a fastener 91 (see FIG. 2). Attachment flange 87 amay include a guide pin 93 a, and attachment flange 87 b may include areceiving aperture 93 b which receives guide pin 93 a to aid in aligningthe two halves of vibration absorber 51 during assembly. Although FIG. 3shows base portion 71, top portion 75, annular adapter ring 77, annulardisk 79, and annular tuning weights 81 as semicircular componentsaligned directly on top of each other, it is preferred that thesecomponents be fastened together in an offset fashion, as is shown inFIG. 2 to ensure that the two halves of vibration absorber 51 aresecurely fastened together. One end of each rod 73 is sunken into andbonded to base portion 71, and the opposing end of each rod 73 is sunkeninto and bonded to top portion 75.

Rods 73 function as springs and are preferably pre-fabricated fiberglasspultruded rods, similar to the composite pultruded rods described inU.S. Pat. Nos. 5,324,563 and 5,462,618, which are incorporated herein byreference as if set forth in full. Each rod 73 is operable between astraight mode in which rod 73 experiences no shear or bending forces,and a deflected mode in which shear and bending moment forces areexerted on rod 73. The number, shape, size, dimensions, materials,arrangement, and spacing of rods 73 may be selectively chosen to tailorthe spring rate and functionality of vibration absorber 51. In thisembodiment, sixteen rods 73 having uniform circular cross-sections areequally spaced apart in a circular pattern around base portion 71 andtop portion 75. Rods 73 are located at a radius R2 from central axis 52.It should be understood, that for embodiments having fewer rods 73,other arrangements may be used, as will be described below with respectto the preferred embodiment of the present invention.

This arrangement of rods 73 and tuning weights 81 provides the requiredstiffness and permits in-plane motion in two degrees of freedom, whileequally distributing the loads in each composite rod 73. Thisarrangement of rods 73 also eliminates pitch and roll rotation whilepermitting in-plane translation. The desired spring rate of vibrationabsorber 51 and an adequate fatigue life of rods 73 is achieved byselectively tailoring the number, location, diameter, and length of rods73. These features minimize the weight and complexity of vibrationabsorber 51 by eliminating the need for having heavy components that arenot utilized. Another advantage of this arrangement is that vibrationabsorber 51 can be quickly and easily observed, inspected, and repaired,if necessary.

Referring now to FIG. 4 in the drawings, a schematic of one rod 73 ofvibration absorber 51 is illustrated. A base end 101 of rod 73 isinserted into and bonded to base portion 71, and a top end 103 of rod 73is inserted into and bonded to top portion 75. Because base portion 71is rigidly coupled to hub 55, base portion 71 in FIG. 4 also representshub 55. In a similar fashion, because top portion 75 is rigidly coupledto tuning weight 81, top end 103 in FIG. 4 also represents tuning weight81. Rod 73 has a length L, a midpoint L/2, and a uniform circulardiameter D. Rod 73 is shown in a deflected mode in which base end 101 isfixed relative to base portion 71 and hub 55; and top end 103, andconsequently tuning weight 81, is deflected a distance δ relative tobase portion 71. FIG. 4 shows rod 73 in the deflected mode shape astuning weight 81 is displaced by in-plane shears and bending moments athub 55.

Referring now to FIG. 5 in the drawings, a free body diagram of rod 73of FIG. 4 is illustrated. Rod 73 is shown separated at midpoint L/2 intotwo beams 73 a and 73 b, with beam 73 a being cantilevered at end 101,and beam 73 b being cantilevered at end 103. Opposing shear forces Pexist at the cantilevered ends of beams 73 a and 73 b. Each cantileveredend is displaced a distance δ/2 from its corresponding fixed end 101 and103.

Referring now to FIG. 6 in the drawings, using elastic beam theory, thederivation of the equation for the spring rate k of each beam 73 a and73 b of FIG. 5 is illustrated. As is shown, the spring rate k for beams73 a and 73 b is a function of diameter D, length L, and the modulus ofelasticity E.

Referring now to FIG. 7 in the drawings, the derivation of the equationfor the allowable bending stress σ_(allowable) for beams 73 a and 73 bof FIG. 5 is illustrated. As is shown, the allowable bending stressσ_(allowable) for beams 73 a and 73 b is a function of diameter D,length L, deflection δ, and modulus of elasticity E. Reorganization ofthe equation for allowable bending stress σ_(allowable) provides theequation for the maximum allowable length L for rod 73 for any givendiameter D, deflection δ, modulus of elasticity E, and allowable bendingstress σ_(allowable). Thus, while it would be desirable to make length Las small as possible to minimize aerodynamic drag, the minimumrequirement for length L is dictated by fiber stresses due to bending.

Referring now to FIG. 8 in the drawings, a schematic of one end of rod73 is illustrated. In this figure, rod 73 has a circular cross-sectionalarea A and a diameter d. An important consideration in the configurationof the present invention is the attachment of rods 73 to base portion 71and top portion 75. It will be appreciated that the end moment at eachend of rod 73 is reacted as a couple. The shear load generated by such acouple results in peak shear stress at the mid-plane through thethickness and produces an interlaminar shear failure of the laminate.One way to minimize these stresses is to selectively tailor the depth Wof base portion 71 and top portion 75 and the geometry of mounting holes111 bored into and/or through base portion 71 and top portion 75. As isshown, mounting hole 111 passes completely through base plate 71, androd 73 is inserted into mounting hole 111 from one side of base plate 71and passed through mounting hole 111, such that rod 73 is flush with theother side of base plate 71. Mounting holes 111 may be countersunk oneach side of base plate 71 and on each side of top plate 75.

Referring now to FIG. 9 in the drawings, a free body diagram of end 101of rod 73 and base plate 71 of FIG. 8 is illustrated. Rod 73 is shown asa beam 73 c having an arbitrary length and being cantilevered at end101. Width W of base plate 71 is represented as a distance x. Opposingshear stresses V are exerted on beam 73 c at the each side of base plate71.

Referring now to FIG. 10 in the drawings, using elastic beam theory, thederivation of the equation for the minimum distance x, which representsthe minimum width W of base plate 71, for the beam arrangement of FIG. 9is illustrated. As is shown, if the allowable interlaminar shear stressis τ_(allowable), the minimum distance x is a function of diameter d,length L, deflection δ, modulus of elasticity E, and τ_(allowable).

Dynamically, vibration absorber 51 is tuned for approximately 3/revvibration in the rotating system by tailoring the spring rate of rods 73and tuning weights 81. This provides reduction in 4/rev vibration in thefixed system, i.e., the non-rotating system. The desired torsionalfrequency is achieved by controlling radius R1 for tuning weight 81 andradius R2 for rods 73.

Because passive vibration absorbers by nature are excited by basemotions, it is important to consider vibration shapes. Referring now toFIG. 11 in the drawings, a plot 121 showing the vibratory mode shape ofmast 61 for the embodiment FIGS. 2 and 3 is illustrated. The horizontalaxis of plot 121 represents horizontal displacement of mast 61, and thevertical axis of plot 121 represents the waterline in inches along mast61, or the vertical height along mast 61 from a datum point, such as theground. The vertical line M at the origin of the horizontal axisrepresents mast 61 in an undeflected shape. For plot 121, the rotorplane of hub 55 is located at point H, which is near waterline 115inches.

In general, rotor hubs are excited by multiple forces and moments, eachwith varying phases relative to each other. Each force or momentproduces its own characteristic vibration shape. For example, a typical4/rev forced response of mast 61 due to in-plane hub shear isrepresented by curve B, and a typical 4/rev forced response of mast 61due to hub moments is represented by curve C.

As is shown, curve B crosses vertical line M at one point N1, and curveC crosses vertical line M at two points N2 and N3. Thus, point N1represents a node point on mast 61 at which there is no deflection inmast 61 due to in-plane vibratory hub shear force, and points N2 and N3represent node points on mast 61 at which there is no deflection in mast61 due to vibratory hub moments. Consequently, there is no anti-node forcurve B along the represented height of mast 61, and there is oneanti-node AN for curve C. Because a vibration absorber located at a nodepoint for a particular vibration shape will not be excited by theassociated excitation, that vibration absorber will not absorb anyvibration. Thus, although a vibration absorber placed at node N1 mayabsorb a small amount of the vibration due to hub moments, i.e. curve C,it will not absorb any vibration due to in-plane hub shear forces.Likewise, although a vibration absorber placed at either nodes N2 or N3,may absorb some vibration due to in-plane hub shear forces, it will notabsorb any vibration due to hub moments. Vibratory mode shapeschematics, such as plot 121, allow engineers to locate the optimumlocation to place vibration absorbers along rotor system drive masts.

The optimum placement of a rotor system vibration absorber is at alocation where it can absorb vibrations from both in-plane shear forcesand bending moments. For the exemplary rotor system represented in plot121, placing a vibration absorber above the rotor hub, i.e., above pointH, is not effective in treating hub moment C, because the vibrationabsorber would be too close to hub moment node N3. In contrast, if thevibration absorber is located near an anti-node, the maximum vibrationabsorption will occur. For the exemplary rotor system represented inplot 121, it would be very effective to place a vibration absorber belowthe rotor hub, because such a location would be close to the hub momentanti-node AN, and would be able to absorb both vibration due to in-planehub shear forces and hub moments.

It should be understood that plot 121 is for a single arbitrary rotorsystem, and that the placement of vibration absorbers will vary greatlyfrom one application to another. The present invention allows thevibration absorber to be placed either above or below the rotor plane,wherever the vibration absorber is most effective in treating theresulting airframe vibration. For some systems, the vibration absorberwill be most effective placed below the rotor hub, and for other rotorsystems, the vibration absorber will be most effective placed above therotor hub, as is the case in the preferred embodiment of the presentinvention.

Referring now to FIGS. 12 and 13 in the drawings, the preferredembodiment of a rotor system 129 having a vibration absorber 130 for ahelicopter or other rotorcraft according to the present invention isillustrated. FIG. 12 is a simplified top plan view schematic showing thepreferred arrangement of rods 131 about a rotor hub 133, and FIG. 13 isa side elevation view in partial cross section of rotor system 129 andvibration absorber 130. Hub 133 is coupled to a drive mast 135 that isdriven by a conventional drive means (not shown). Vibration absorber 130is operably associated with rotor system 129, such that a plurality ofrods 131 are disposed about rotor hub 133 in the open spaces betweenrotor blades 137 that are pivotally coupled to hub 133 via lugs 134. Inthe preferred embodiment, one rod 131 is disposed between each pair ofadjacent rotor blades. A base member 139 is rigidly coupled to a housing138 that is rigidly mounted to the underside of hub 133. Thisarrangement eliminates any moment rotation at base member 139. Rods 131are coupled at one end to base member 139 and at the other end to anupper plate 136.

One or more tuning weights 159 are coupled to upper plate 136 in arecessed portion 160. It should be understood that tuning weights 159and upper plate 136 may be integrally combined into a single component.Upper plate 136 and tuning weights 159 are cantilevered at the upperends of rods 131 and are free to deflect through a selected distance. Inother words, upper plate 136 and tuning weights 159 serve as the mass,while rods 131 serve as the spring in spring-mass vibration absorber130.

Upper plate 136 includes a downwardly extending cup portion 151 that isoperably associated with drive mast 135 to provide a fail safe means inthe event that one or more rods 131 fail during operation. Cup portion151 includes at least one interior over-travel stop 153 that isconfigured to engage a cap 155 disposed atop drive mast 135. Over-travelstops 153 restrict the deflection of rods 131 and prevent vibrationabsorber 130 from damaging rotor system 129 should one or more rods 131fail during operation. An upper housing 161 is coupled to and extendsabove hub 133. An aerodynamic canopy 163 is coupled to upper housing 161to reduce the aerodynamic drag caused by vibration absorber 130. Upperhousing 161 and aerodynamic canopy 163 are not coupled to vibrationabsorber 130 and do not affect the vibration absorption functions ofvibration absorber 130.

As described above, the spring rate k of vibration absorber 130 isheavily dependant upon the number, length, location, elastic modulus,and diameter of composite rods 131. If rods 131 are too thin, the strainis too high and their fatigue life is too short. If rods 131 are tooshort, their stiffness is too high. If rods 131 are relatively thin,then more rods 131 are needed to provide an adequate spring rate k. Forfour-bladed applications, it is preferred that four rods 131 havingtapered lengths be used.

Rods 131 are preferably pultruded composite rods similar to thecomposite rods described above. However, instead of having a uniformcross-sectional diameter, each rod 131 is preferably machined or moldedto taper inwardly, such that the longitudinal profile of each half ofeach rod 131 is in the shape of a non-linear function with the minimumcross-sectional area A1 being located at the longitudinal midpoint ofeach rod 131. In the preferred embodiment, the non-linar function is acubic function. In addition, rods 131 may be covered with a layer ofglass fabric to minimize surface delamination. Because shear forces aregreatest at the smallest cross-sectional area, the hub shears arecarried at the midpoint A1 of each rod 131. Although the midpoints ofrods 131 lie in the rotor plane of hub 133, it should be understood thatthe midpoints of rods 131 may be located at various heights dependingupon the vibration absorption desired.

Each rod 131 has a longitudinal axis 144 that is located at a radius R3from a longitudinal axis 132 of mast 135. Because the transverse shearforce P in FIG. 5 remains constant over the length of rod 131, theminimum cross section in the middle of rod 131 is sized for shear stressτ_(allowable) from FIG. 10. Moving away from the middle of rod 131toward ends 141 and 142, a moment PL/2 in FIGS. 5 and 10 is added toshear force P; therefore, the cross-sectional area A is increased tocarry the combined shear and bending moment loads. Thus, the optimumtapered shape of rods 131 follows a cubic function along axis 144 whichoptimizes the structural strength integrity and weight, and meets thedesired stiffness and fatigue requirements.

Rods 131 include lower end portions 141 and upper end portions 142 thathave increased cross-sectional diameters. This allows the shear force Pand the bending moment PL/2 to be transferred from rod 131 to basemember 139 at one end, and from rod 131 to the moving mass, i.e., upperplate 136, at the other end. Each rod 131 is held in place within amounting hole 145 in base member 139 by one or more wedge members 143that bear against the thickened lowered end 141 of each rod 131. In asimilar manner, each rod 131 is held in place within a mounting hole 147in upper plate 136 by one or more wedge members 149 that bear againstthe thickened upper end 142 of each rod 131. The moments PL/2 arereacted at ends 141 by the tapered wedge shaped surfaces of wedgemembers 143, and at ends 142 by the tapered wedge shaped surfaces ofwedge members 149. Adequate clamp-up is provided at each end 141 and 142to preclude fretting in the joint in spite of the high oscillatoryloading. This unique non-linear taper-shaped configuration andtaper-clamped joint mounting arrangement of rods 131 provides almostinfinite fatigue life for rods 131.

The configuration of rods 131, upper plate 136, and tuning weights 159provides the required stiffness and permits in-plane motion in twodegrees of freedom, while equally distributing the loads in each rod131. This arrangement also eliminates pitch and roll rotation whilepermitting in-plane translation. The desired spring rate and an adequatefatigue life of rods 131 is achieved by selectively tailoring the shapeof rods 131. This configuration minimizes the weight and complexity ofvibration absorber 130 by eliminating the need for having heavycomponents that are not utilized. Another advantage of this arrangementis that vibration absorber 130 can be quickly and easily observed,inspected, and repaired, if necessary.

It is apparent that an invention with significant advantages has beendescribed and illustrated. Although the present invention is shown in alimited number of forms, it is not limited to just these forms, but isamenable to various changes and modifications without departing from thespirit thereof.

1. A rotorcraft comprising: a fuselage; a drive means carried by thefuselage; a rotor system including a rotor hub and rotor blades, therotor system being coupled to the drive means; and a spring-massvibration absorber comprising: a base member coupled to the rotorsystem; a tuning weight; and a plurality of elongated rods disposedbetween the base member and the tuning weight; wherein the rods serve asthe spring and the tuning weight serves as the mass such that vibrationfrom the rotor system is absorbed by the oscillatory deflection of therods and the tuning weight.
 2. The rotorcraft according to claim 1,wherein the spring rate of the vibration absorber is determined byselectively tailoring the number, location, size, and shape of the rods.3. The rotorcraft according to claim 1, wherein the vibration absorberabsorbs vibratory hub shear forces.
 4. The rotorcraft according to claim1, wherein the vibration absorber absorbs vibratory hub moments.
 5. Therotorcraft according to claim 1, wherein the vibration absorber absorbsboth vibratory hub shear forces and vibratory hub moments.
 6. Therotorcraft according to claim 1, wherein the vibration is the principalblade-passage frequency.
 7. The rotorcraft according to claim 1, whereinthe rods are composite rods manufactured from a unidirectional compositefiber reinforced material.
 8. The rotorcraft according to claim 7,wherein the composite rods are covered with a composite fabric tominimize delamination.
 9. The rotorcraft according to claim 1, whereinthe rods have a uniform cross-sectional geometry.
 10. The rotorcraftaccording to claim 1, wherein the rods have a non-uniformcross-sectional geometry.
 11. The rotorcraft according to claim 10,wherein the rods have a longitudinal profile in the shape of anon-linear function.
 12. The rotorcraft according to claim 11, whereinthe non-linear function is a cubic function.
 13. The rotorcraftaccording to claim 1, wherein the base member is coupled to the rotorsystem above the rotor hub and the tuning weight is disposed above therotor hub.
 14. The rotorcraft according to claim 1, wherein the basemember is coupled to the rotor system below the rotor hub and the tuningweight is disposed below the rotor hub.
 15. The rotorcraft according toclaim 1, wherein the base member is coupled to the rotor system abovethe rotor hub and the tuning weight is disposed below the rotor hub. 16.The rotorcraft according to claim 1, wherein the base member is coupledto the rotor system below the rotor hub and the tuning weight isdisposed above the rotor hub.
 17. The rotorcraft according to claim 1,further comprising: a second vibration absorber comprising: a secondbase member coupled to the rotor system; a second tuning weight; and asecond plurality of elongated rods disposed between the second basemember and the second tuning weight; wherein vibration from the rotorsystem is also absorbed by deflection of the second plurality of rods.18. The rotorcraft according to claim 17, wherein the base member iscoupled to the rotor system above the rotor hub and the tuning weight isdisposed above the rotor hub; and wherein the second base member iscoupled to the rotor system below the rotor hub and the second tuningweight is disposed below the rotor hub.
 19. A vibration absorber for useon a rotorcraft having a rotor system including a drive means, a drivemast coupled to the drive means, a rotor hub coupled to the drive mast,and rotor blades pivotally coupled to the rotor hub, the vibrationabsorber comprising: a housing adapted for mounting to the underside ofthe rotor hub; a base member coupled to the housing; a tuning weightdisposed above the rotor hub; and a plurality of rods, each rod beingcoupled at one end to the base member and coupled at the other end tothe tuning weight; wherein the vibration absorber absorbs both vibratoryhub shear forces and vibratory hub moments generated by the rotorsystem.
 20. The vibration absorber according to claim 19, wherein onerod is disposed between each pair of adjacent rotor blades.
 21. Thevibration absorber according to claim 19, further comprising: an upperplate disposed between the rods and the tuning weight; wherein the upperends of the rods are coupled to the upper plate, and the tuning weightis coupled to the upper plate.
 22. The vibration absorber according toclaim 21, further comprising: a travel stop means disposed on the upperplate and operably associated with the drive mast to prevent thevibration absorber from damaging the rotor system in the event offailure of the vibration absorber.
 23. The vibration absorber accordingto claim 19, wherein the rods are composite rods manufactured from aunidirectional composite fiber reinforced material.
 24. The vibrationabsorber according to claim 23, wherein the composite rods are coveredwith a composite fabric to minimize delamination.
 25. The vibrationabsorber according to claim 19, further comprising: a canopy disposedover the vibration absorber to reduce aerodynamic drag generated by thevibration absorber.
 26. The vibration absorber according to claim 19,wherein each rod comprises: an elongated body portion manufactured froma unidirectional fiber reinforced pultruded composite material; and alongitudinal profile in which each half of each rod is in the shape of anon-linear function, such that the ends have enlarged cross-sectionalareas at the couplings to the base member and the tuning weight, and thesmallest cross-sectional area is located at the longitudinal midpoint ofeach rod; whereby the fatigue life of each rod is increased and thegreatest vibratory hub shear forces are located at the longitudinalmidpoint of each rod.
 27. The vibration absorber according to claim 26,wherein the non-linear function is a cubic function.
 28. The vibrationabsorber according to claim 19, wherein the blade-passage frequencyvibration is reduced.
 29. A composite spring-mass assembly comprising: afixed body; at least one elongated composite rod attached at one end tothe fixed body, the composite rod being manufactured from aunidirectional fiber reinforced pultruded composite material; a movablemass attached to the other end of the composite rod, such that themovable mass is cantilevered relative to the fixed body; wherein thespring rate of the composite spring-mass system is determined byselectively tailoring the number, arrangement, and shape of the rods.30. The composite spring-mass system according to claim 29, wherein eachend of each elongated composite rod is tapered in the shape of anon-linear function, such that both ends of each composite rod haveenlarged cross-sectional areas and the longitudinal midpoint has thesmallest cross-sectional area.
 31. The composite spring-mass systemaccording to claim 30, wherein the non-linear function is a cubicfunction.